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Tuesday, June 15, 2010

Duramax Diesel Fuel Systems: Electronic Components

Since 1994, GM diesel engines have used electronic engine controls to monitor and provide more precise diesel fuel injection. In the 16 years that have passed since then, diesel engine fuel injection strategies have advanced dramatically. This has enabled manufacturers to nearly double performance output and still reduce overall emissions. A whole new approach to diesel fuel injection was introduced by the first Duramax in 2001 (Ford had introduced electronic common rail diesel fuel injection before that, however the system required engine oil pressure to actuate the injectors. The fuel system used on the first Duramax was strictly electronically actuated and, as a result, far more precise). The Bosch common rail direct fuel injection system was new to the North American market at the time and provided speed and precision that had not previously been available. These factors, among others, allowed the Duramax engine to set the high-water mark for diesel engine power output. The 2001 Duramax engine rated at 300 HP and 520 foot-pounds of torque. Power output has increased incrementally to the current 365 HP and 660 foot-pounds of torque in the LMM Duramax.

In the first article of this series, we reviewed the basic mechanical components of the Duramax fuel system, their locations and functions and common failure points. In this article we will continue with an overview of the electronic components of the Duramax fuel system, their locations and functions and common failure points. As you go through this article you should begin to appreciate the overall complexity of the Duramax fuel system. The most sophisticated part of the fuel system is the Engine Control Module and its software. When you consider how much information the ECM has to process in real-time and then calculate an output on the timescale of microseconds (millionths of a second), it becomes readily evident why the Duramax engine has set the bar for diesel engine fuel systems. Let's begin our overview of the electronic portion of the Duramax diesel fuel system.

Note: This article is written to build a basic framework for Duramax owners to be able to perform diagnostics on their own vehicles, so it's not exactly recreational reading. It presumes that one is familiar with basic electrical theory. However, even if the reader is not that technically inclined, this series of articles will provide insight into common failure points that will help the less technically inclined Duramax owner to communicate more effectively with a service technician.

Computers, Inputs and Outputs

The brains of the whole Duramax fuel system is the Engine Control Module (ECM). With the Duramax/Allison combination, there is a separate Transmission Control Module (TCM) required to operate transmission functions; the TCM communicates closely with the ECM. TCM operation will not be considered in detail in this article: suffice it to say that it relies on the ECM for proper information in order to operate the Allison transmission correctly.

The ECM is located at the front of the engine compartment, just below the driver's side battery. In the early Duramax engines, there was a separate Fuel Injector Control Module (FICM), located on the front passenger side of the engine. The FICM was required to provide the high voltage, high current drive for the fuel injectors. As both injector and ECM technology advanced, the FICM became unnecessary. For the 2006 and newer Duramax engines, the ECM was able to drive the injectors by itself.

The ECM is a full-blown computer, with a robust set of electronic hardware to ensure its reliability. ECM failures are an exception, not the rule. The ECM processes data that it receives from various sensors and sometimes by other computer modules and then produces a programmed output. It contains a method of translating external inputs into digital signals, much like a home computer translates the inputs of the keyboard and mouse. It contains a central processing unit, where the information processing actually takes place. It also contains a couple of types of memory:

RAM style memory for processing duties and ROM style memory to store the various operating parameters and the operating system.

The ROM memory is also known as EEPROM, or Electronically Erasable Programmable Read Only Memory. This is the memory that a power programmer device will read and write to in changing operating parameters.

It is also the memory that is updated periodically when GM releases new software to improve diagnostics or operation of a vehicle.

Once the information is processed, the ECM will command various outputs in response, much like the data output of a conventional computer is output to a monitor or printer.

When you power up a conventional computer, it runs through a power-on self test, basically an internal diagnostic of its various systems. The ECM on a Duramax does something similar, however it continues to run diagnostic routines throughout the time that it is being powered up and sometimes in sleep mode when the ignition has been shut off.. These diagnostic routines help identify any failures with inputs, outputs or with the ECM itself that could create problems with engine operation. Beginning in 1996, Federally mandated OBD II regulations required a common system of reporting diagnostic results. Any kind of failure that could make the vehicle produce more than 1.5 times the normal expected emissions was to be reported to the driver of the vehicle by turning on the Malfunction Indicator Lamp. As we will discuss later, there are a number of enhancements in how a modern ECM reports a failure that can help a technician to get down to the root of a problem reasonably quickly in most cases.

On Allison automatic transmission equipped trucks, a total of three modules were required to operate the engine and transmission from 2001 to 2005. These three modules, the ECM, TCM and FICM all communicated with one another on a dedicated communication bus, known as a Controller Area Network, or CAN bus. CAN bus was used because the traditional GM communication bus in 2001 was unable to manage the data transport speeds that were required between these modules. When injector pulsewidths are determined in millionths of a second, you can be sure that communication between these modules needed to be blazing fast. The CAN bus remains even after 2006 to communicate between the ECM and the TCM, again because real-time data speeds are required between these modules.

As Direct Fuel Injection strategies advanced, more computer power was required in the Duramax. On the LB7, a variation of the standard Delphi ECM was used. On the LLY, a more powerful Delphi ECM was introduced (versions of this ECM were adopted some time later for gasoline engines). On the LBZ and newer engines, a new and very powerful Bosch ECM is being used. Variations of the Bosch ECM are now also used on GM's gasoline direct-injection engines.

The Basics of ECM Input Sensing

Various sensors are required to provide input to the ECM. Some sensors have more influence with regard to engine operation, others have more influence on the operation of emissions devices. The ECM needs to have reliable data from these inputs in order for the engine to operate smoothly. A number of driveability concerns can be traced back to failed sensors or damaged wiring.

The sensors on the Duramax engine can be divided into categories. Each sensor utilizes two or three wires to operate and will either provide a digital input or an analog voltage input to the ECM.

Digital Input: A digital sensor will provide a voltage signal that is either fully off or fully on.

Analog Input: An analog sensor will provide a varying voltage signal depending on the measured condition.

AC frequency Input: This is a basic AC signal generator. The output voltage generally varies with frequency, however the ECM will only count the frequency as its input signal.

Two-wire sensor: A two-wire sensor uses one wire to provide a voltage to the sensor and another wire to return a signal to the ECM. They can be further subdivided:

Pull-up sensor, which is a sensor that modulates a voltage that is provided to the ECM.

Pull-down sensor, where the ECM provides voltage to the sensor that the sensor modulates to ground.

Three-wire sensor: A three wire sensor has one wire that provides a reference voltage to operate the sensor that is delivered by a module or ignition input, a separate ground wire and a signal wire that returns a voltage to the ECM.

Consider an overview of the various input sensors used on the Duramax engine (abbreviations are provided where applicable).

ECM Input Sensors

Crankshaft Position Sensor (CKP)

The crankshaft position sensor is located on the lower passenger side of the front engine cover. It is a three wire Hall-effect sensor that provides a digital signal. This sensor reads a 57-tooth reluctor wheel located on the nose of the crankshaft behind the front engine cover. Essentially the reluctor wheel is a steel plate with 57 notches cut in its circumference. As the teeth pass by the magnetic field of the crankshaft position sensor, a digital signal is generated and passed on to the ECM. This allows the ECM to see crankshaft revolution in three degree increments. The ECM needs to resolve three degrees of crankshaft revolution for misfire diagnostics and to properly balance the engine idle speed. Ordinarily, the reluctor would contain 60 notches, however a larger notch was provided to give the ECM a reference point for top dead center of cylinders one and four. The crankshaft position sensor is absolutely crucial to engine operation. Without this sensor the ECM has no reference point to know at which point is should fire the injectors. If the signal is interrupted in any way, the ECM will not trigger the fuel injectors.

Camshaft Position Sensor (CMP)

The camshaft position sensor is located near the crankshaft position sensor. It is bolted to the front of the engine cover. It is also a three-wire Hall effect sensor, however it reads a reluctor plate on the front of the camshaft. This sensor assists the ECM to determine which cylinders are at top dead center. The ECM uses this sensor initially when cranking. Once the ECM has identified which cylinders are coming to top dead center, it no longer requires this sensor for any further input.

Engine Coolant Temperature Sensor (ECT)

The engine coolant temperature sensor is located in the thermostat housing. It is a two wire sensor that provides an analog signal. It is placed in direct contact with the engine coolant. Information provided by the ECT is used to assist in determining injection timing, glow plug control, intake air heater control and cranking fuel control. On LLY and newer Duramax engines this sensor also has an influence on turbocharger boost and EGR operation.

Fuel Rail Pressure Sensor (FRP)

The fuel rail pressure sensor is mounted in various locations depending on the generation of Duramax engine. On the original LB7 Duramax it is located in the fuel junction block. On the LLY Duramax, it is located in the middle of the right-hand fuel rail. Since 2006 the fuel rail pressure sensor has been located at the rear of the right-hand fuel rail. It is a three-wire sensor that provides an analog signal. This sensor is a crucial input for proper fuel delivery. The ECM requires precise measurements of fuel pressure in order to calculate the injector pulsewidth. This sensor is also used to diagnose low fuel pressure conditions.

Fuel Temperature Sensor

The fuel temperature sensor is mounted at the rear of the left-hand return line. It is a two wire sensor that provides an analog voltage signal. It monitors actual fuel temperature and allows the ECM to compensate injection timing and various other parameters for fuel temperature. It is also used to determine if there is a fuel overtemperature condition so that it can limit fuel rail pressure in order to protect the fuel system.

Mass Airflow Sensor (MAF)

The mass airflow sensor is mounted on the air filter assembly. It is a three wire sensor that provides an analog voltage signal. It measures the mass of air entering the engine, normally expressed in pounds per minute or grams per second. This sensor is used to adjust fueling in order to limit black smoke. MAF data is also used in turbocharger diagnostics and EGR operation.

Intake (Ambient) Air Temperature Sensor (IAT, IAT1, or AAT)

The intake air temperature sensor is located in the same housing as the mass airflow sensor. It is a two wire sensor that provides an analog voltage signal. It measures inlet air temperature at the airbox and has a bearing on injection timing diagnostics and turbocharger operation.

Intake (Manifold) Air Temperature Sensor (IAT2, or IAT)

Since 2006 the Duramax has also included a manifold air temperature sensor. It is located in the center distribution intake manifold on the engine itself. It is a two wire sensor that provides an analog voltage signal. This sensor is useful in determining the efficiency of the intercooler and protecting the engine from damage in extreme operating conditions. It has an influence on injection timing and boost pressure control.

Manifold Pressure Sensor (MAP)

The manifold pressure sensor is located in the intake manifold and measures pressure delivered by the turbocharger. It is a three wire sensor that provides an analog voltage signal. It is primarily used for diagnostic purposes in the first LB7 Duramax engine. Since the LLY Duramax the manifold pressure sensor has been used almost exclusively for turbocharger control. ECM uses feedback from the sensor to adjust turbocharger vane position to deliver desired boost.

Barometric Pressure Sensor

The barometric pressure sensor is located above the left-hand valve cover and it is used to measure atmospheric pressure. It is a three wire sensor that provides an analog voltage signal. Barometric pressure is used by the ECM to compensate injection timing and boost control.

Vehicle Speed Sensor (VSS)

The vehicle speed sensor is located on the transmission housing on two wheel drive trucks or on the rear of the transfer case on four-wheel drive trucks. It is a two wire sensor that provides a varying AC frequency signal. The vehicle speed sensor is used primarily by the transmission control module to calculate shift points, but it is also used for the ECM for diagnostic calculations as well as to provide input to the speedometer..

Turbocharger Vane Position Sensor

On LLY and newer Duramax engines the turbocharger is controlled by a variable-vane system instead of a wastegate. The ECM needs to know the exact angle of the vanes in order to effectively modulate the turbocharger's operation. The Turbocharger Vane Position Sensor is locate on the top of the turbocharger housing. It is a three wire sensor that provides an analog voltage signal. The turbocharger vane position sensor provides ECM with feedback to assist in boost control as well as diagnostics.

EGR Position Sensor

On the LLY and newer Duramax engines, the EGR position sensor is internal to the EGR solenoid. It is a three wire sensor that provides an analog voltage signal. It is used to determine the exact position of the EGR piston valve. The ECM modulates the EGR position signal in order to achieve the desired position using feedback from the sensor. This sensor is also used for EGR diagnostics and to determine if the EGR valve is sticking.

EGR Cooler Sensor 1 and Sensor 2 (EGR EGT1 and EGR EGT2)

On the LMM and newer Duramax engines, the EGR cooler contains two sensors to measure exhaust gas temperatures. These are two wire sensors that provide an analog voltage signal. One sensor is located at the rear of the EGR cooler and one sensor is located at the front of the EGR cooler. These inputs are used by the ECM to assist calculating proper EGR flow and operation. These sensors are separate from and not to be confused with the EGT sensors located in the exhaust system of the LMM diesel (see next item).

Exhaust Gas Temperature Sensor 1 and 2 (EGT1 and EGT2)

On the LMM and newer Duramax engines, the exhaust system has two exhaust gas temperature sensors. One is located behind the catalytic converter, the other is located behind the diesel particulate filter. These are two wire sensors that provide an analog voltage signal. These sensors are used to monitor the temperature of the catalytic converter output as well as the temperature of the diesel particulate filter during a regeneration cycle. Input from the sensor is used to regulate injection timing, throttle valve position, turbocharger boost and post injection to control regeneration temperature.

Diesel Particulate Filter Differential Pressure Sensor

On the LMM and newer Duramax engines, there is a compound sensor similar in operation to the manifold pressure sensor but specially designed to measure the difference in exhaust gas pressure across the diesel particulate filter. The differential pressure sensor is located on the frame underneath the pickup box just behind the cab of the truck. This is a three wire sensor that provides an analog voltage signal. The ECM uses this sensor to monitor the soot buildup in the diesel particulate filter. After using input from the sensor and comparing that information with a complex internal soot model calculation, the ECM can determine if there is a need for a regeneration cycle.

Throttle Position Sensor (TPS)

On the LMM and newer Duramax engines, there is a throttle valve assembly that is used to regulate engine airflow during regeneration. The throttle valve assembly is bolted to the front of the inlet tube below the intake air heater. The assembly contains a three wire sensor that provides an analog voltage signal corresponding to the throttle valve angle. The ECM uses the throttle position sensor in order to determine the exact position of the throttle valve.

Accelerator Pedal Position Sensor (APP)

The accelerator pedal position sensor is the means by which the driver can provide a desired torque input to the ECM. It is located at the top pivot of the accelerator pedal assembly. From 2001 to 2005 the APP sensor housing contained three sensors for redundancy. Since 2006 the APP assembly contains two sensors, which continue to provide reliable redundancy. Each of these sensors is a three wire sensor that provides an analog voltage signal. Having more than one sensor, the ECM can compare each signal and provide a failsafe in case one sensor should fail. Generally, if one or more sensors fail, the ECM will set a diagnostic trouble code and the engine will go into reduced engine power mode. The accelerator pedal position sensor assemblies are extremely reliable and generally do not cause any problems.

Water-in-Fuel Sensor

The water-in-fuel sensor is located at the bottom of the fuel filter housing. It is a two wire sensor that provides a digital on or off voltage to the ECM. Normally this sensor is used only to warn the driver of water being present in the bottom of the fuel filter housing. The ECM reads the input of this sensor and passes along the information to the instrument panel cluster.

Low Oil Level Sensor

The low oil level sensor is located in the lower oil pan assembly. It is a two wire sensor that provides a digital on-or-off signal to the ECM. Normally this sensor is used only to warn the driver of a low engine oil condition. The ECM reads the input of this sensor and passes along the information to the instrument panel cluster. In certain instances, for example medium duty trucks, this sensor can also be tied to an algorithm ECM to shut down the engine for engine protection.

Low Coolant Level Sensor

The low coolant level sensor is located in the bottom of the coolant surge tank assembly. It is a two wire sensor that provides a digital on or off signal to the ECM. This sensor again is typically used to warn the driver of a low coolant level condition. The ECM will read the input of this sensor and pass along information to the instrument panel cluster. In some cases, as in medium duty trucks, this sensor can be used by the ECM to shut down the engine for engine protection.

Fuel Level Sensor(s)

The fuel level sensor is located inside the fuel tank on the fuel pickup module. It is a two-wire sensor that provides an analog voltage signal to the ECM The ECM will read the signal from this sensor and interpret it as a fuel level percentage. It then transmits this data to the Instrument Panel Cluster for the fuel gauge. . If the truck is equipped with dual fuel tanks, this sensor works in tandem with a second sensor in the rear fuel tank,.In this case , the ECM will activate a secondary fuel pump to transfer fuel from the rear tank to the front tank when the fuel in the front tank gets below a certain threshold. The ECM always tries to maintain the same fuel level in both tanks. Fuel pickup to the engine is strictly from the front tank at all times. The ECM averages the two fuel level sender signals and provides the average fuel level to the Instrument Panel Cluster.

The Basics of Output Devices

ECM will generally use two basic methods to control output devices. One way is by switching on and off the ground path for the output device, the other way is by switching on and off the voltage source to the output device. A circuit that switches the ground path for a device is known as a low-side driver circuit. A circuit that switches the voltage source for a device is known as a high-side driver circuit. The most common method of controlling output is to use a low-side driver. The ECM will typically only use a high-side driver to operate a device if vehicle or occupant safety may be compromised by a circuit fault.

We can break down these two methods of control into subcategories. One method is to simply turn the device on or off; the other method is to control a device by means of pulsewidth modulation (PWM). Pulsewidth modulation allows the ECM to vary the average electrical current flow of a particular device. Used most commonly on the Duramax, PWM enables the ECM to precisely control the mechanical position or strength of a specific output device. Consider the following overview of the various output devices controlled by the ECM.

ECM Output Devices

Fuel Rail Pressure Regulator (FRPR)

The fuel rail pressure regulator is an electric solenoid device mounted on the back of the high-pressure pump. It controls available fuel to the high-pressure pump section, effectively controlling the fuel pressure delivered by the high-pressure pump. The ECM uses a high-side driver with pulse width modulation to vary electrical current to the solenoid. The ECM will increase the duty cycle of the pulse width modulated signal to reduce fuel pressure, conversely it will decrease duty cycle in order to increase fuel pressure. If this solenoid is disconnected, fuel pressure will automatically go to its maximum. This allows the engine to run even if there is an electrical failure of this circuit. The ECM uses several parameters including engine RPM, engine coolant temperature, desired fuel rate and fuel temperature to determine the desired fuel pressure. The ECM monitors actual fuel rail pressure using the fuel rail pressure sensor and adjust the fuel rail pressure regulator output in order to match the calculated desired fuel pressure.

Fuel Injectors

The fuel injectors are also pulse width modulated solenoid devices. They are located underneath the rocker arm covers on the first generation Duramax: they are mounted externally on newer Duramax engines. The fuel injectors are numbered according to their respective cylinders. On a Duramax the number one cylinder is located on the passenger side front of the engine and the passenger side cylinders are all odd-numbered, 1-3-5-7. The driver's-side cylinders are even-numbered, 2-4-6-8. The LB7 and the LLY Duramax engines used an intermediary module called the Fuel Injection Control Module or FICM to actuate the fuel injectors. The ECM did not control the fuel injectors directly, although it pulsed a specific injector output for each injector that was hardwired to the fuel injection control module. Since 2006, the ECM has operated each fuel injector directly. In all cases, a low-side driver was used either in the FICM or the ECM to control the fuel injectors.

The actual operation of the fuel injectors is similar for all generations of Duramax engines, only the voltage requirements to actuate the injector have changed. The length of the on-time of each injector is calculated by the ECM from the actual fuel pressure and the quantity of fuel desired. A mode known as pilot injection is also used on all Duramax engines. Pilot injection delivers a small shot of fuel into the combustion chamber prior to the main injection event, in order to precondition the combustion chamber and smooth the combustion pressure curve. A mechanical delay is always present, known as hysteresis, so the ECM will always electrically turn on the injector for slightly longer than the injector is actually mechanically open.

Turbocharger Vane Control Solenoid

Starting with the 2004 LLY, the Duramax has been equipped with a Variable-Vane style turbocharger. This kind of turbocharger does not use a wastegate to limit boost pressure. Instead, it uses variable-angle vanes placed around the exhaust turbine to direct exhaust gasses onto the turbine at varying angles. This kind of turbocharger is typically more responsive and more efficient at generating boost over a wider range of engine operation than a typical wastegated turbocharger. It also allows more control over exhaust backpressure to assist devices such as the EGR valve. It can be used as an exhaust brake as well.

A hydraulic piston is used to control the angle of the vanes. This hydraulic piston is driven by engine oil pressure, which is directed to either side of the piston by the Turbocharger Vane Position Control Solenoid mounted on the side of the turbocharger. The ECM uses a high-side driver which provides a pulsewidth modulated signal to this solenoid to control oil flow to the hydraulic piston, which in turn controls turbocharger vane angle.

Exhaust Gas Recirculation (EGR) Valve

The exhaust gas recirculation valve is a solenoid device that operates a piston which controls that controls the mixing of intake and exhaust gasses. Since the 2004 LLY, it is located on the passenger front side of the engine. The ECM controls the EGR valve with pulsewidth modulation. It varies the current delivered to the EGR valve to achieve the desired valve position based on feedback from the EGR valve position sensor. An increase in duty cycle opens the valve more, smaller duty cycle allows the valve to close. The ECM calculates desired EGR valve operation based on several factors including engine temperature, boost pressure and delivered fuel quantity.

Glow Plugs

On the early LB7 Duramax, the glow plugs were energized by a relay which was directly controlled by the ECM. The ECM used a high-side driver to energize the glow plug relay and turn the glow plugs on. The ECM calculates the glow plug on time based on engine coolant temperature and ambient temperature.

On LLY and newer Duramax engines, the glow plugs are controlled by a separate Glow Plug Control Module (GPCM). The ECM communicates appropriate information to the GPCM which directly controls the glow plugs individually. The GPCM uses pulse width modulation to control current flow to these newer glow plugs. The GPCM is capable of performing diagnostics on each individual glow plug and will communicate faults to the ECM.

Intake Air Heater

On the original LB7 Duramax, there was also an intake air heater. It was located in the center intake manifold pipe. This intake air heater was energized by a relay located in the glow plug relay housing. The intake air heater relay was controlled by a low-side driver in the ECM. The ECM would use engine coolant temperature and ambient temperature to determine desired intake air heater operation.

Since 2006, the Duramax has featured a slightly different intake air heater, located above the EGR valve assembly. This grid style intake air heater is also electronically controlled by the GPCM, using data communicated from the ECM. The intake air heater has a high current switching device built into its housing that is controlled by the GPCM. The GPCM will also communicate fault information regarding intake air heater to the ECM.

Wait To Start Lamp

The wait to start lamp in the instrument panel is directly controlled by the ECM. The ECM uses a low-side driver to turn the light on or off. This light indicates to the driver to wait for glow plug preheating of the combustion chambers before starting the engine.

Fuel Transfer Pump

On dual fuel tank equipped vehicles, the ECM will use a high-side driver to control a relay that energizes a fuel transfer pump. The fuel transfer pump is located on the driver's side frame rail above the rear differential assembly. The ECM monitors the fuel level in each fuel tank and will energize the transfer pump to balance the fuel levels. The ECM energizes this relay by providing a voltage to the relay, in turn the relay provides a voltage to the fuel pump.

Throttle Valve (TV)

A Throttle Valve has been included on the 2007 and newer LMM Duramax engine. It is located ahead of the Intake Air Heater. It's function is to manage total airflow through the engine during a regeneration cycle for temperature control of the catalytic convertor and diesel particulate filter. The ECM controls the throttle valve using a special circuit that can provide forward or reverse current flow through the TV motor. The polarity of the circuit depends entirely on whether the ECM wants to drive the motor to open the valve or drive the motor to close the valve. It uses pulsewidth modulation to hold the TV motor and valve in a specified position.

Malfunction Indicator Lamp (MIL)

The malfunction indicator lamp, sometimes known as the service engine soon lamp, is directly controlled by the ECM. ECM provides a controlled ground circuit to illuminate this lamp when a fault is detected that can cause an increase in emissions of more than 1.5 times the specified limit. This light is provided only to warn the driver that the vehicle is producing excess emissions, nothing more. The ECM can detect some faults that do not push the engine past it's emissions limit, store a diagnostic trouble code and not illuminate the malfunction indicator lamp. Some faults can even affect driveability and the ECM will not illuminate the malfunction indicator lamp. Therefore, not having an illuminated MIL is not a reliable indication that there is no fault or no stored diagnostic trouble codes with the vehicle. On the other hand, if the MIL is lit, it does not necessarily mean that there is something wrong that will create a driveability problem.

Duramax LMM Engine Components

Diagnostic Trouble Codes (DTC) and What You REALLY Need to Know

Since 1996, OBD-II law mandated that manufacturers report emissions faults using a common protocol. Hence we have a standardized system of numbering trouble codes and far more detail than what was previously available to assist in diagnostics. The numbering system is fairly straightforward and not necessary to cover here. DTC's are only required to set by law when the vehicle exceeds 1.5 times the mandated emissions level, however manufacturers like GM have used DTC's to report much more fault information to us than is strictly mandated by law. More importantly, it is necessary to have a clear understanding of how trouble codes can be used to report a fault in order to be effective in diagnosing various problems that may be indicated by the trouble code. Trouble codes come under these categories:

Voltage Fault DTC's These are DTC's related to main system voltages. For example, the voltage supplied to the ECM can be low, which will obviously affect the ECM's operation. This can also apply to a reference voltage circuit that powers one or a group of sensors. These DTC's only relate to power circuits. An example is P0641 5 Volt Reference 1 Circuit. Often there will be a group of other Component level DTC's set at the same time, the root cause being the reference voltage that powers a group of sensors. Therefore, Voltage Fault DTC's need to be diagnosed before the Component level DTC's.

Programming or Software Performance Fault DTC'sThese are DTC's that relate to a module not being configured correctly. An example is P160C Engine Calibration Not Programmed – GPCM. Usually these pop up after an ECM has been replaced and not configured properly. These are rarely if ever seen under normal operation.

Component Fault DTC's

These are codes that relate to a specific sensor or output device. Be careful of the terminology on these: As an example - P0193 Fuel Rail Pressure Sensor Circuit High Voltage - this is not a Voltage Fault DTC, it merely indicates that the signal voltage from the Fuel Rail Pressure Sensor is high.

System Fault DTC's These are DTC's that relate to the performance of a particular system. An example is P0299 Turbocharger Engine Underboost. System performance can be affected by a failed component, hence any Component Fault DTC's should be diagnosed first.

Now those categories may be rather confusing to follow at first, but they are important. They establish the hierarchy with which to proceed with diagnostics. Some failures may induce several trouble codes and affect more than one system. How you get down to the root problem, sorting through these layers of trouble codes, is determined by the above hierarchy, starting with the category 1 Control Module Fault DTC's, if they are present. This may be easier to explain by example:

If you have an ECM with 10 different trouble codes in it, some related to the EGR valve, some related to the Turbocharger, some related to Fuel Rail Pressure Regulator and one code that indicates an internal ECM fault, which one would you look at first? By far the most likely root cause of failure, in this case, is a malfunctioning ECM. If you have a malfunctioning ECM, it can report all kinds of weird faults in several different unrelated systems causing a huge list of trouble codes. But the root failure is the ECM itself, reported by a trouble code like P0606 Control Module Internal Performance. Imagine if a technician spent hours trying to find the cause of an EGR performance trouble code when the EGR system itself is working just fine! A technician can waste a great deal of time chasing secondary symptoms of the real fault if he does not follow this DTC hierarchy. For your convenience, this article includes a master DTC list for all Duramax engines produced up to 2010. GM's diagnostics are all set up to presume this hierarchy to start with, however most aftermarket scanners and diagnostic information do not make this obvious, which can lead many an owner or technician down the garden path, so to speak. Therefore one needs to be aware of this before attempting diagnostics.

What determines whether or not the ECM will set a trouble code depends greatly on the software that GM provides in the ECM. Known as diagnostic routines, DTC's will set after the ECM goes through a set range of checks on a system or components. For some DTC's the diagnostic routine is simple: If the ECM loses the signal completely from a specific sensor for a prescribed period of time, a component level DTC sets. An example of this is P0118 Engine Coolant Temperature Sensor High Voltage. If the sensor is unplugged, the voltage that the ECM sends out to be modulated by the ECT sensor goes to it's highest level, in this case, 5 volts. The ECM expects to see a voltage between 0.5 to 4.5 for a properly functioning sensor, so when the ECM reads 5 volts on this sensor circuit for longer than 10 seconds, P0118 will set. If we see a P0118, then we know we have the relatively simple task of finding an electrical fault either in the wiring, connector, or the ECT sensor itself.

Some DTC's use a far more complex diagnostic routine - the engine needs to be running for a set period of time in a certain condition at a certain temperature and certain outputs need to be commanded to a certain level, then the diagnostic may run and compare certain inputs with a complex internal calculation of what it expects to see. This kind of complex diagnostic routine is typical of a System Fault DTC. An example of this is the DTC P0299 Turbocharger Engine Underboost. The engine has to run within a specific condition of temperature and time and if the boost is 35KPa below the threshold determined by an internal table, it will set the code. System DTC's can be more difficult to diagnose because of the very specific conditions that are needed to duplicate the problem and the fact that many different failures can be the cause the specific system to not perform as expected. In the P0299 example, everything from the intake to the exhaust to a mechanical failure within the turbo itself can generate this code. Additionally, one has to consider the possibility that the diagnostic routine itself may not be set up correctly and will set a trouble code even though no real fault is present. This is one of the main reasons why GM continues to send out software updates for ECM's - to improve the sensitivity of or correct faults in specific diagnostic routines

Most often, the customer will not actually be informed of a software update unless there is a crucial emissions or reliability problem - GM will send out a recall notice for these kinds of updates. Most often, a customer will get their ECM updated when they bring the truck in for service and complain about a specific condition or DTC that the update addresses. Unfortunately, besides recalls, there are no other ways of notifying the customer of such updates. The customer has to discover the fault for themselves and bring the truck in.

Common Electrical Faults That You Should Know About

With every generation of Duramax engine, there are common failure points electrically. These failures are typically electrical in nature and can involve wires or terminals. Other common failures include faults within sensors themselves, or conditions that cause a sensor to fail. Please note that some of these failures are common to what was reported in the first article of this series. Mechanical failures will not be considered in this overview, they have already have been addressed in the previous article. The following is a comprehensive overview of the common electrical failures over the years of the Duramax engine's production. Most have GM Bulletins associated with them.

As you can see by the title of the Bulletin, this problem can cause a whole raft of DTC's to set and cause all kinds of different concerns. The basic problem here may be a terminal backing out of one of the two main engine connectors located on the driver's side of the engine. I have also noted over the years that the mounting screws that hold the connectors to the engine may rub through the wiring on the lower connector, causing various different electrical concerns. With any random problem, checking the status of these connectors and the related wiring nearby is a good first visual check.

This bulletin is an example of a System DTC setting in a strange condition that the engineers may not have thought about. Some of us in the North - actually - most of us in the North country use block heaters. I don't look at the temperature before going to bed to determine whether or not I should plug in the block heater - if it's cold it's cold! GM wrote the diagnostic routine based on the fact that you shouldn't start using the block heater until the temperature goes below 0°F. The solution to these two DTC's setting is to install a block heater that has a built in thermostat. This switches the block heater on only when it falls below 0°F ambient.

This bulletin refers to wire chaffing at the wire harness near the Fuel Injection Control Module. This can cause assorted injector electrical DTC's. The solution is to remove the wire tie that holds the wire harness to the FICM bracket itself, repair the wires and keep the wires away from that bracket.

Common on the LLY Duramax engines, injector terminal fretting can set these DTC's. The solution is to completely replace the fuel injector connectors for injectors #2 and #7 with a newly designed 90 connector and support brackets. Terminal fretting is caused by tiny movement of the connector, causing the electrical contact surfaces on the terminal to abrade and build up oxidization or debris, limiting the effectiveness of the terminal connection.

Closely related to the previous Bulletin, this is also a terminal fretting problem with the fuel injector connectors. This is occurs much less commonly than the previous bulletin and in this case, the solution is to simply replace the connector end.

Duramax Diesel Winter Airbox

This problem is basically outside contamination of the Mass Airflow Sensor, causing it to deliver incorrect Airflow data. The cause is simple, snow and ice ingestion into that air filter. The solution is to perform a winter modification to the airbox lid and use the winter front.

This is a procedure that needs to be performed at the dealership. In involves a software update and an EGR cleaning procedure initiated by the GM Tech II scan tool.

2007-10 LMM P0546, P2033 EGT 1 and 2 Sensors High Voltage

The EGT sensors in the exhaust system of the LMM Duramax can be subject to vibration damage under certain conditions and the sensing element will literally break. The solution is to replace the affected sensor.

2006-07 LLY, LBZ Various Trouble Codes, Driveability Faults

Duramax Diesel Wiring Harness Rub Issue

If you have several uncommon or strange trouble codes setting, a good place to look is the ECM wire harnesses that lay on the Driver's side inner fender well. These can build up debris underneath, rub through and corrode several wires going to the ECM. Repair the wires and re-route them to prevent rubbing on the inner fender well.

Most commonly when the engine surges after it is warmed up, the fuel pressure regulator is sticking. The solution is to replace the fuel pressure regulator. In extreme cases, a sticking fuel pressure regulator can cause the fuel rail pressure to spike, open the high-pressure blow-off valve and set low fuel rail pressure codes.

2006 LLY, LBZ, LMM, MAP sensor Icing Or P0106

This is another problem that can persist in cold weather. The tip of the MAP sensor will literally ice off, blocking the sensor from reading an accurate manifold pressure. Typically P0106 will set, however it is possible to see P0299 Turbocharger Engine Underboost with this as well. One solution can be to perform the winter airbox modification as shown above. A more permanent solution is to perform a special modification to the MAP sensor itself, which will be detailed in a future article.

There can be a few problems that lead to these trouble codes setting and they need to be isolated before changing any components. However, I have found that the most common cause is a failed Turbocharger Vane Position sensor.

Building a Diagnostic Framework

Your average reader is not likely to get too deep into diagnosing specific issues with their own Duramax. However, some Duramax owners will roll up their sleeves and try to solve their own problems. If you are one of the latter, you'll likely purchase your own scan tool or use a software package like EFILive or HPtuners to read trouble codes at the very least. This article is designed to help you get started in understanding basic ECM operation and diagnostic procedures. The first article in this series provided an overview of the mechanical components of the fuel system along with common issues.

As time progresses, we planned to add hyperlinks to the diagnostic trouble code table presented in this article and link them to specific Duramax Issues and Answers articles that deal with the particular diagnostic trouble code. There you will find additional information that will help you to track down any faults with your Duramax. In each Duramax Issues and Answers column from here on, there will be a detailed and easy to follow diagnostic procedure presented for the more common DTC's. When readers have access to some form of scan tool, it is recommended that they include the Diagnostic Trouble Codes with any issues and answers they may send in for the magazine's consideration. As more and more of this information comes together, maxxTORQUE magazine will become a more powerful resource to help you to keep your Duramax in top shape.